Field of the Invention
The present invention relates to tools for use in a wellbore. More particularly, the invention concerns a check valve for an injection string.
Prior and Related Art
As used herein, a wellbore is a wholly or partially cased borehole through a geological formation. In the following description and claims, “uphole” refers to the direction toward the surface, regardless of the inclination of the wellbore with respect to the vertical. Similarly, “downhole” refers to the opposite direction. That is, “downhole” does not mean “within the wellbore” in the following description and claims.
During stimulation and hydraulic fracturing, an injection string is inserted into a wellbore while a fluid is circulated downhole through the string and returned uphole through an annular space between the string and the wellbore. The string is provided with an injection valve placed between uphole and downhole isolation packers. When the packer and valve assembly is positioned at the zone to be stimulated, the pump rate is increase. This increases a bore pressure within the string and causes the isolation packers to set and the injection valve, e.g. a sliding sleeve valve, to open. The injection may require a bore pressure in the order of 70 bar (1000 psi) over the normal pressure at the rock face, i.e. the pressure before injection. Integrated assemblies for performing the above operations are described in previous patent applications assigned to the assignee of the present disclosure.
A further development is described in NO20150652A1 assigned to the present assignee. The subject matter of this application includes a mechanically operated (MO) valve assembly uphole from the pressure activated packer and valve assembly to flush sand and debris away from the annular space around the string after injection, such that the pressure activated valve and packer assembly may be moved to another location or retrieved. The MO valve assembly may comprise an MO packer uphole from an MO valve. Both MO devices are operated by manipulating the string from the surface, in particular by sequences of down-weights, right-hand-turns and pull-ups. Thus, the mechanically operated devices operate independent of the pressure within the string. The MO-devices require an anchor to fix a downhole part of the string with respect to the wellbore.
During injection and/or flushing, there is a need to avoid reverse flow, e.g. kicks as known in the art. Hence, it is beneficial to include a check valve in the injection string. However, a check valve is subject to corrosion and/or abrasion from the injection fluids used in stimulation and fracturing operations.
The objective of the present invention is to provide an improved check valve that solves or reduces at least one of the problems above while retaining the benefits of prior art.